Electricity produced by a solar cell is expensive due to high solar cell module cost. In order to significantly reduce the cost of solar electricity, it is desirable both to increase cell efficiency as well as to significantly reduce the costs of PV module fabrication.
Copper ternary chalcogenide compounds and alloys are promising light-absorber materials for solar cell applications due to their direct (and tunable) energy band gaps, very high optical absorption coefficients in the visible to near-infrared (IR) spectrum range and high tolerance to defects and impurities. The methods used for preparing light absorption layer of Copper indium-gallium-selenium/sulfur (CIGS) thin film solar cells can be categorized into two classes: (1) high-vacuum vapor deposition method (thermal evaporation, and magnetron sputtering) and (2) non-vacuum liquid phase method (spraying, printing and electro-deposition).
CIGS thin film solar cells have been recognized as the next generation of solar cells. CIGS solar cells have the advantages of low cost, high efficiency, long-term stability, superior performance under weak illumination, and desirable resistance to radiation. However, commercial mass production of reliable CIGS thin film solar cells has been challenging because of the complicated conventional process for preparing the light absorption layer of CIGS thin film solar cells, leading to a low yield rate and a high production cost.
The CIGS thin films having a small area that is prepared by vacuum vapor deposition methods possess excellent quality, and the corresponding solar cells can exhibit very high photoelectric conversion efficiencies. As disclosed by the US National Renewable Energy Laboratory (NREL), a highest efficiency of 19.9% has been achieved with a copper-indium-gallium selenium thin film solar cell with an effective area of 0.419 cm2 prepared by the three-stage co-evaporation process. Refer to Ingrid Repins, Miguel A. Contreras, Brian Egaas, Clay DeHart, John Scharf, Craig L. Perkins, Bobby To and Rommel Noufi, “19.9% efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor”, Prog. Photovolt: Res. Appl. 16 (2008) 235, for useful background information.
However, it would be difficult to ensure the uniformity of thin films when these methods are used for the deposition of thin film solar cells having a large area. Moreover, various factors such as low yield rate resulting from the complexity of those processes, high capital investment, low raw material utilization rate and low productivity, leads to a very high production cost, which prohibits the mass production of CIGS thin film solar cells by these methods.
It is desirable to achieve substantial cost reduction when using non-vacuum liquid phase methods and large area thin films can be conveniently deposited. Various low cost non-vacuum liquid phase methods were developed for the preparation of light absorption layer of CIGS thin films.
Electro-Deposition:
U.S. Pat. No. 4,581,108 discloses a method utilizing a low cost electro-deposition approach for metallic precursor preparation for a two-stage processing technique. In this method a Copper (Cu) layer is first electrodeposited on a substrate. This is then followed by electro-deposition of an In layer forming a Cu/In stack during the first stage of the process. In the second state of the process, the electrodeposited Cu/In stack is heated in a reactive atmosphere containing Se forming a CuInSe2 compound layer.
More recently, U.S. Pat. No. 2010/0140101 A1 discloses a method including electrodepositing a film stack of Cu, In, Ga, then a Cu—In binary alloy film followed by a electro-deposition of Se layer, reacting the precursor stake form an absorber layer. Low cost, high utilization rate of raw materials and facile deposition of large area thin films are typical advantages of electrochemical deposition method. However, very large gaps existing between reduction potentials of Cu, In and Ga often bring about enrichment of copper, great difficulties in the stoichiometry control and high concentration of impurities in the produced thin films. Subsequent modification of the stoichiometry of thin films by PVD/electro-deposition is usually necessary, which leads to a sharp increase in production cost.
Spray Pyrolysis:
Spray Pyrolysis is a cost efficient method to prepare CIGS thin films. However, high concentration of detrimental impurities, high roughness and un-uniformity in large area thin films hindered the practical utilization of this method. It is further very difficult to prepare CIGS thin films qualified for the photovoltaic devices by spray pyrolysis, and solar cells prepared by this process show extremely low photoelectric conversion.
Ink Printing:
Non-oxide-based non-vacuum liquid phase method was developed by Nanosolar corp. for preparing CIGS thin films (see U.S. Pat. No. 7,306,823). This method comprises the following steps: (1) preparing nanoparticles or quantum dots of copper or indium or gallium or selenium; (2) coating the surface of nanoparticles or quantum dots with one or more layers of copper, indium, gallium, and selenium, etc. wherein the stoichiometry ratios between different elements in the coated nanoparticles are controlled by adjusting the composition and thickness of the coating layer; (3) dispersing the coated nanoparticles in a solvent to produce a slurry; (4) forming a precursor thin film from the slurry by a non-vacuum process such as printing, etc.; and (5) short annealing the precursor film to produce the targeted CIGS thin films.
Low cost, high utilization rate of raw materials, applicability of flexible substrates and facile deposition of large area thin films can be readily achieved by this method. However, since nano-particles are used in this method, and parameters of the coated nanoparticles, such as particle size, size distribution, surface morphology and stoichiometry are very hard to be precisely controlled, thus resulting in unfavorable controllability, high complexity and poor reproducibility of the process.
Prior art methods for producing CIGS thin films exhibits low-throughput and expensive due to use of final high temperature annealing process or another, which hampers the large-scale commercialization of CIGS thin film solar cells. It is desirable to develop a novel method for producing CIGS thin films that can overcome the disadvantages described hereinabove, and is highly applicable to the industrialization of CIGS thin film solar cells.
An innovative process approach that offer high throughput, large area uniformity, and inexpensive roll-to-roll compatibility is needed.